The present invention relates to the inspection of nuclear fuel rods, and assemblies of fuel rods, to detect and locate defective rods.
The large nuclear reactors utilized for power generation consist of an array of a large number of fuel rods containing nuclear fuel. Each rod comprises a metal tube which may be from 8 to 15 feet long and approximately 1/2 inch in diameter, and which contains a stack of cylindrical fuel pellets of suitable fissionable material such as uranium oxide. The upper end of the tube is empty of fuel pellets and forms a plenum for a gas or other fluid under substantial pressure which fills the top of the rod and also the small clearance space which is provided around the pellets to allow for expansion or swelling. The fuel rods are supported in parallel groups in fuel assemblies which may typically contain from 49 to as many as 300 fuel rods, and the complete nuclear reactor is made up of a large number of these fuel assemblies arranged in a suitable configuration.
The metal tubes of the fuel rods constitute the primary containment boundary for the radioactive nuclear fuel, and inspection to verify the integrity of the rods is of primary importance. In the manufacture of the fuel rods, the tubing itself and the end cap welds are carefully inspected and helium leak tests of the completed rods are also performed. Since a nuclear reactor may contain as many as 40,000 fuel rods, however, it will be apparent that a significant probability exists that some number of defective tubes will be present even with a highly effective manufacturing quality control program. Furthermore, even initially good fuel rods may develop cracks, pinholes or other defects in service and such defective rods must be detected.
The reactor is usually shut down about once a year for refueling and at this time, as well as during initial installation, the fuel rods must be inspected to detect any defective rods that may be leaking fission products. The reactor and the fuel assemblies are immersed in a pool of water during the refueling operations and during removal of the fuel assemblies for replacement or inspection. In the standard method of inspection which has been used heretofore, the flow of water through each fuel assembly to be inspected is blocked so as to allow the fuel rods to heat up, which causes expulsion of fission products into the water through any defects that may exist in the rods. The water is then checked to detect the presence of radioactivity in the water, indicating that such leakage of fission products has occurred and that a defective rod is present in the assembly being tested. This system requires that the water be pumped to a sampling station and repeatedly analyzed or checked for radioactivity and is a very slow procedure requiring as much as an hour for each fuel assembly. Complete inspection of all fuel rods in a large power reactor is, therefore, very time consuming and may require as much as a week during which the reactor is out of service. This represents a very high cost to the operator of the reactor and a more rapid but simple and reliable inspection method is needed.
In my copending application Ser. No. 595,577, filed July 14, 1975 as a continuation of Ser. No. 376,697, filed July 5, 1973, now abandoned there are disclosed and claimed a method and apparatus for inspecting nuclear fuel rods in a much more rapid and inexpensive but highly reliable manner. This method consists essentially in causing a change in the relative pressures of the fluid within the fuel rods and the water in which they are immersed such that the internal pressure within the rods becomes higher than the external water pressure. If any defect such as a crack or pinhole exists in any of the fuel rods, the result is the expulsion of a bubble or bubbles of fluid or other material from the interior of the fuel rod into the liquid. Sensing means are provided to detect the emission of such bubbles to provide a reliable and instantaneous indication of the presence of the defect and also of the location of the defect. This provides a very sensitive and reliable method of inspection, but it is possible for some of the fluid in the rod to be trapped or adsorbed by the fuel pellets, particularly at the top of the rod, and in some cases such trapped fluid is not expelled and defects in the upper part of the rod may not be detected.